Within a digital library cell, channel length scaling can be achieved by increasing the size of the poly. For example, a portion of a 65 nm library cell 10 is illustrated in FIG. 1. The library cell 10 is provided on a 5 nm GDS grid 12 and includes a poly element 14 and two contact elements 15. Channel length scaling is achieved by providing 5 nm of additional poly elements 16 to both sides of the poly element 14, thereby increasing the size of the poly by 10 nm. This increase in size results in channel length scaling of approximately 17%. By adding poly elements 16 to both sides of the poly element 14, symmetry is maintained. FIG. 2 illustrates leakage and performance curves for devices which incorporate channel length scaling of 17%.
It is more difficult to achieve channel length scaling for 45 nm digital libraries than for 65 nm digital libraries. FIG. 3 illustrates an attempt to use the channel length scaling method described above, in connection with a 45 nm digital library cell. The 45 nm digital library cell 30 includes a poly element 32 and two contact elements 34. Channel length scaling is achieved by providing 5 nm of additional poly elements 36 to both sides of the poly element 32, thereby increasing the size of the poly by 10 nm. This increase results is channel length scaling of 25%. By providing additional poly elements 36 on both sides of the poly element 32, symmetry is again maintained within the cell, however, a 25% channel length scale is too aggressive to be useful, would not be acceptable from a performance standpoint, and would not be competitive with the rest of the industry.
Channel length scaling in excess of 10% may lead to unwanted threshold voltage mismatch. This large shift in the threshold voltage is due to the threshold roll up/off curve. Large channel length scaling may result in additional stress induced mobility/performance variation due to contact to poly spacing.
The positioning of contacts impacts channel stress. While positioning contacts relatively close to a channel results in low stress at the channel, positioning contacts further from a channel results in greater stress within the channel.
As contact-to-poly spacing increases, the longitudinal stress generally remains constant until the contact-to-poly spacing reaches approximately 0.14 um. As contact-to-poly spacing increases, the vertical stress increases exponentially until the contact-to-poly spacing reaches approximately 0.14 um.
The force applied by the strain nitride film is proportional to the volume and proximity of the film to the channel. The film volume and proximity is modulated by the poly-to-poly distance; the contact dimension and contact-to-poly distances; and the contact pitch.
Due to the negative effects of large channel length scaling, an alternative method for channel length scaling is needed.